This paper reports an improvement in the development of a low-cost QPI microscope offering new capabilities in term of phase measurement accuracy for label-free live samples in the longer term (i.e., hours to days). The spatially separated scattered and non-scattered image light fields are reshaped in the Fourier plane and modulated to form an interference image at a CCD camera. The apertures that enable these two beams to be generated have been optimised by means of laser-cut apertures placed on the mirrors of a Michelson interferometer and has improved the phase measuring and reconstruction capability of the QPI microscope. The microscope was tested with transparent onion cells as an object of interest.
This paper reports the progress to develop a practical phase measuring microscope offering new capabilities in terms of phase measurement accuracy and quantification of cell:cell interactions over the longer term. A novel, low cost phase interference microscope for imaging live cells (label-free) is described. The method combines the Zernike phase contrast approach with a dual mirror design to enable phase modulation between the scattered and un-scattered optical fields. Two designs are proposed and demonstrated, one of which retains the common path nature of Zernike’s original microscopy concept. In both setups the phase shift is simple to control via a piezoelectric driven mirror in the back focal plane of the imaging system. The approach is significantly cheaper to implement than those based on spatial light modulators (SLM) at approximately 20% of the cost. A quantitative assessment of the performance of a set of phase shifting algorithms is also presented, specifically with regard to broad bandwidth illumination in phase contrast microscopy. The simulation results show that the phase measurement accuracy is strongly dependent on the algorithm selected and the optical path difference in the sample.
Multi-wavelength interferometry (MWI) has a long tradition in the field of optical metrology and is used as a
solution to a number of applications. MWI phase unwrapping procedures are usually based on beat
wavelength approaches, Chinese Remainder Theorem (CRT) techniques, or the method of Excess Fractions
(EF). Each of these unwrapping approaches have a distinct advantage for a given application: Beat
wavelength and CRT based approaches offer a direct calculation of integer fringe order, and EF offers many
alternative sets of wavelengths to achieve a large unambiguous measurement range (UMR) with high
reliability. Nevertheless, a drawback of Beat wavelength and CRT based approaches is that they have a
limited UMR due to the available measurement wavelengths, and the alternative approach the EF is often
impractical in practice, because the calculation of the integer fringe order involves a large number of
computational steps. Recently, we have reported a unified theory of beat wavelength, EF and CRT
approaches, which enables the derivation of phase unwrapping approaches with low computational effort,
which hitherto had only been possible for CRT and beat wavelength approaches, whilst offering flexibility in
choosing the measurement wavelengths for a given UMR, which had previously only been the case for EF. In
this work, we briefly summarize the previous developed framework that determines the UMR and
measurement reliability and derive optimization criteria that are based on parameters, which are dependent
on the choice of the measurement wavelengths. The developed optimum wavelength selection strategies
maximize the dynamic range of interferometer for a given value of phase noise the dynamic range of
A novel method is presented to simultaneously measure shape and color information of artifacts containing color features. The technique operates by projecting composite red, green, and blue fringe patterns onto the surface of colorful objects. Theoretical analysis proves that there is no crosstalk between color channels during phase calculation by phase-shifting algorithm when three fringe patterns with the same fringe number are coded into red, green, and blue channels to form a composite RGB fringe pattern image. The color channel giving the maximum modulation depth at each pixel is used to measure shape information. Since three color channels are used, color information of the object surface can be extracted with high dynamic range from the same fringe pattern images. Using the recently developed color fringe projection system, composite RGB fringe patterns are projected onto colorful objects to test the proposed method. The experimental results show that the range of colors that can be measured and that shape and color information of colorful objects can be reliably obtained.
Multi-wavelength interferometry (MWI) has a long tradition and provides a solution to a number of applications in the
field of optical metrology. In MWI phase unwrapping procedures are usually based on beat wavelength approaches,
Chinese Remainder Theorem (CRT) techniques, or the method of Excess Fractions (EF). Each of these unwrapping
approaches has distinct advantages making it suitable for a given application. Beat wavelength and CRT based
approaches offer a direct calculation of integer fringe order, however, the unambiguous measurement range (UMR) is
limited by the available measurement wavelengths. On the other hand, EF offers many alternative sets of wavelengths to
achieve a large UMR with high reliability; however, the calculation of the integer fringe order involves a large number of
computational steps. In this work, a unified theory of beat wavelength, EF and CRT approaches is reported. It is shown
that the calculation of the integer fringe order requires a low computational effort, which hitherto had only been possible
for CRT and beat wavelength approaches, whilst offering flexibility in choosing the measurement wavelengths for a
given UMR, which had only been the case for EF. As the model can be used in a predictive way to determine the UMR
and measurement reliability it is possible to define optimization criteria that are based on parameters which are
dependent on the choice of the measurement wavelengths.
Interferometric metrology is well established for both single point and full field measurements.
However, absolute techniques for long range measurements, spanning 100's to 10,000's of
fringe orders whilst maintaining sub-fringe resolution have been reported with widely varying
levels of performance. In this paper, techniques for long range multi-wavelength
interferometry are reviewed with respect to applications of classical interferometry and fringe
projection profilometry. Whilst hierarchical geometric series methods provide a potential
solution it is shown that significantly greater freedom in wavelength selection is obtained by
applying excess fraction principles and a new predictive model for this technique is discussed.
Phase-based 3-D fringe projection imaging systems have been widely studied because of the advantages of fullfield,
high accuracy, fast acquisition and automatic processing. The calibration of phase-based 3-D systems is an
important procedure, which builds up the relationship between the obtained absolute phase map and the depth
information. In this paper, a fast and flexible calibration method for phase-based 3-D imaging systems is presented
based on an uneven fringe projection method. The relationship between the measured phase and the object's depth
is linear and independent of pixel position, so it is possible to calibrate the 3-D imaging system by using discrete
markers (with known separation) on a white plate. Projecting uneven fringe pattern sets onto the plate can calculate
the absolute phase of each marker. At the same time, the depth of the markers can be obtained by general camera
calibration methods. Therefore, the linear relationship between the measured phase and the depth can be
determined. The proposed method was applied to calibrate an existing phase-based 3-D imaging system which
utilizes the uneven fringe projection technique. The entire calibration procedure does not require accurate
movement of a reference plane within the measurement volume. The calibrated system was evaluated by measuring
an accurately positioned white plate. Experimental results show that the proposed calibration method can easily
build up an accurate relationship between the absolute phase and the depth information.
The method of excess fractions has a long history in metrology. More recently excess fractions has been exploited to
resolve the fringe order ambiguity in interferometric metrology with varying degrees of success. There are a variety of
reports detailing the performance of excess fractions, for example, using 4 wavelengths an unambiguous measurement
range of 2.4 mm was achieved with a phase noise of 1/900th of a fringe. In an independent report a 4 wavelength
interferometer gave an unambiguous measurement range of 17 mm with a phase noise of 1/200th of a fringe. It has
been found that the unambiguous range of an excess fractions multi-wavelength interferometer depends on the
wavelengths used within the system. A theoretical model is reported in this paper that can be used in a predictive way
to determine the unambiguous measurement range based on three wavelength dependent parameters. The excess
fractions model is consistent with beat wavelength techniques but offers many alternative sets of wavelengths to
achieve, for a given phase noise, a particular unambiguous measurement range with a given reliability.
In this paper, we explore the optimization and implementation of multi-wavelength interferometers such
that measurements beyond the largest beat wavelength can be achieved reliably. A hybrid beat wavelength
approach is presented that also exploits wavelength coincidence between two beat wavelengths in order to measure
unambiguously over an extended range. The performance of the approach has been explored both through
simulations and experimental validation has been obtained using a fiber interferometer with 4 measurement
wavelengths. The initial results have demonstrated 1/200th of a fringe phase resolution giving absolute metrology
over 18.16 mm, or a dynamic range of 1 part in 2.4×106.
In this paper, we present an understanding of the failure modes of excess fractions solutions to multi-wavelength
interferometry. From this basis, an approach to select optimum measurement wavelengths has been
introduced. A practical fiber optic sensor has been constructed for simultaneous detection of the intensity at four
measurement wavelengths. The system has been demonstrated using two wavelength selections that are very near
the optimal configuration and the data analyzed using an excess fractions solver. Initial results have shown a
measurement range of 17 mm with reliable and robust absolute metrology from a system with a phase noise of
1/200th of a fringe. This corresponds to an overall dynamic range of 1 part in 2×106.
In this paper, we present a novel method to measure shape and colour information of a colourful object by
projecting separate red, green and blue colour fringe patterns onto the object surface. With regard to the object
surface's colour, the modulation at each pixel position in the three colour channels has different values. For
example, when projecting blue fringe patterns onto a red point, the corresponding pixel has too low a fringe
modulation to accurately calculate the phase (shape) information; but with red fringe patterns a high fringe
modulation is obtained. Therefore, phase information of the red point can be calculated by projecting red fringe
patterns. For each object point, by comparing the modulation values from the three colour channels, it is possible to
choose the channel having maximum modulation, and hence phase information can be reliably obtained by the
phase-shifting algorithm. The fringe order information is obtained by using the optimum three-frequency selection
method, so there is a maximum reliability in determining the fringe order and the 3-D shape of an object with step
or large slopes on the surface. Since three colour channels are used, colour information of the object surface can be
extracted with high dynamic range from the same fringe patterns. Chromatic aberration between colour channels is
unavoidable and can be eliminated by a software-based method. Using the recently developed colour fringe
projection system, separate colour fringe patterns are projected onto a mug having different colour patterns, a
colourful box and plate, and a colour checker card to test the proposed method. The results show the range of
colours that can be measured and that shape and colour information of colourful objects can be reliably obtained.
In this paper, we present a new color fringe projection system based on optimum frequency selection and discuss its implementation. Recent results in optimum frequency selection in temporal phase unwrapping have shown that the resolution limit of commercial data projectors is reached using 3 different sets of projected fringes with different pitch. Therefore, there is a synergy between optimum fringe projection and commercial color (RGB) projectors and cameras offering the potential for parallel data acquisition and simultaneous measurement of surface color parameters. The hardware of the system is comprised of a Digital Light Processing (DLP) video projector, a color 3-chip CCD camera and a personal computer supporting two monitors. The software is developed in Microsoft Visual C++ and OpenGL. The phase calculation algorithm is based on the optimum three-frequency selection, so it has a maximum reliability to determine the fringe order and can obtain 3-D shape of an object with large slope changes or discontinuities on the surface. Since each RGB color channel carries a fringe pattern of a certain pitch, any coupling between the color channels from the projector to the CCD camera affects the phase measurements obtained. Commercially available systems inherently contain crosstalk and we compare some methods to decrease the coupling effect. As absolute fringe order is determined by heterodyning between fringes with different pitch that are imaged in separate color channels, chromatic aberration can cause incorrect calculation of fringe order. We have investigated the most sensitive heterodyne process and the color channels with minimum chromatic aberration to mitigate these effects. We also give a novel software method to further compensate for chromatic aberration. Results show that our system has the advantages of fast acquisition, large dynamic range, robustness in discontinuities, and potential color extraction.
Improved patient comfort and the need for better quality diagnostic information provide the motivation for new sensor development for the urinary tract. Optical sensors based on single mode fibre optics offer unique advantages in terms of access and miniaturization. We report the design, manufacture and evaluation of a diaphragm based sensor to give better than 10 mbar pressure sensitivity. The diaphragm is formed from a medically compatible material and it's geometric parameters set to give the desired resolution. The rear surface of the diaphragm has a thin aluminum coating such that an interference signal can be detected between the light reflected from the diaphragm and the distal end of the fibre. A number of approaches have been investigated for the analysis of the signal from the sensor using broadband illumination where minimizing overall system cost has been a major driver as well as achieving the required performance. A comparison of the techniques is given and experimental data presented with validation of sensor deflection from a white light interferometer.
We present the application of wavefront sensing to 3-dimensional particle metrology for measuring the 3-component velocity vector field in a fluid flow across a volume. The technique is based upon measuring the wavefront scattered by a tracer particle from which the 3-dimensional tracer location can be calculated. Using a temporally resolved sequence of 3-dimensional particle locations the velocity vector field is obtained. In this paper we focus on an anamophic technique to capture the data required to measure the wavefront. Data is presented from a reconstruction of the phase of the wavefront as well as from a more pragmatic approach that examines only the defocus of that wavefront. The methods are optically efficient and robust and can be applied to both coherent and incoherent light in contrast to classical interferometric methods. A focus of this paper has been the filtering techniques in order to reliably extract the particle images from the overall image field. The resolution and repeatability of the depth (or range) measurements have been quantified experimentally using a single mode fiber source representing a tracer particle. A first proof of principle experiment using this technique for 3-dimensional PIV on a sparsely seeded gas phase flow is also presented.
We present two novel multi-frequency techniques for absolute range measurement in interferometry. The first employs a generalised optimum multi-frequency technique for maximising the measurement range by careful selection of the measurement frequencies. Furthermore, the approach utilises the minimum number of measurement wavelengths and hence reduces the complexity of an interferometer for a given application. The second technique uses the same measurement frequency selection approach as in the well-known technique of excess fractions, but introduces a novel algorithm based on the Chinese remainder theorem to produce reliable fringe order information in the presence of phase measurement noise. A comparison of these techniques with the method of excess fractions has been performed by computer simulation showing that the generalised optimum multi-frequency approach offers the best combination of process speed and measurement reliability. The new techniques have been successfully applied to data from coherent and incoherent fringe projection systems to produce shape data. The results from an initial investigation into a fibre interferometer for high speed single point ranging with 2 measurement frequencies is also presented.
We present two novel multi-frequency techniques for absolute range measurement in interferometry. A comparison of these techniques with the method of excess fractions has been performed by computer simulation and experimental data is presented.
Electronic Speckle Patterin Interferometry (E.S.P.I.) is a holographic interferometry technique technique that can be used to measure out-of- plane deformations and vibration amplitudes up to a few microns with an accuracy of a few nanometres, in near real time. Two-wavelength holographic contouing is an adaptation of this technique that utilises two lasers in an E.S.P.I. system, generating fringes with spacing determined by the frequency difference between the two lasers. Careful choice of laser wavelengths enables us to measure the shape of an object on a scale and accuracy that is more useful in engineering applications. In addition, by measuring the shape of an object before and after the application of a static load, deformation can be measured on this same enginering scale. The engineering drive for this technology is to gain high spatial resolution modal information on light structures (e.g. body panels) with a non-contacting measurement device. The shape and vibration data can then be mapped to each other on a pixel to pixel basis, giving over a quarter of a million data points containing position, vibration amplitude and vibration phase information. This data can then be used to create hybrid Finite Element/Experimental models or be used to predict the noise radiation levels from a vibrating object. In the system described here the 2 laser wavelengths are applied simultaneously giving the capability to obtain the object shape in an engineering environment, i.e. away from a stable optical table. The model data is obtained in a similar manner and therefore the complete system is operable in engineering laboratories.
An optical, fiber-based, speckle shearing interferometer is described. The instrument uses a highly birefringent optical fiber to illuminate a test object with equal intensities of light guided by the orthogonal polarization eigenstates of the fiber. A Wollaston prism is used to obtain two sheared images with adjustable shear. Optical phase changes between the sheared images are readily achieved, without mechanical movement of components, by straining the optical fiber. Object strain determination, by fringe analysis with phase stepping techniques, is readily achieved. Vibration analysis by heterodyning is also reported.
This paper presents a combined measurement system based on TV Holography for vibration measurement and structured light for surface form measurement. A common CCD camera is used for both measurements. In both cases phase stepping techniques have been developed for automation of the analysis. The vibration measurements have been achieved using an 8 frame stereoscopic algorithm which can analyze complex vibrational modes. The two data sets are combined in a wireframe representation and animated to provide a direct visual interpretation of the information. It is necessary to measure complex vibration modeshapes and the surface form of a structure in order to predict noise radiation. Noise radiation predictions are of increasing importance in the design and optimization of motor vehicle structures to achieve desired levels of refinement. Noise radiation can be measured directly using the appropriate microphones and analysis equipment. However, such techniques provide no direct information on the source of the noise, or provide an insight for an approach to reduce the noise level.
The application of fringe analysis has enhanced the application of all optical analysis techniques to industry. Emphasis has traditionally been placed upon such issues as the accuracy of the algorithm used, the number of frames required and the speed of processing. However, the practical use of such techniques at Rover has resulted in a more pragmatic view of this technology and the opinion that other issues dominate its successful use in industry. This paper presents these views by relating experience of applying fringe analysis to TV Holography, Moire, and Photoelastic systems.
Holographic interferometry has been used in a large scale transonic wind tunnel to produce a 3D flow visualization. The experiments have been carried out on a model civil transport aircraft wing and turbine powered engine simulator combination. This study is significant industrially as the method forms a diagnostic for turbofan installations. The holograms show many relevant flow features including shock waves, flow interactions between the engine simulator flow and the freestream flow, secondary flows, and acoustic waves. Quantitative 3D position information has also been obtained for some of these features. A comparison to other flow diagnostic methods has been made in this paper.
Application of particle image velocimetry (PIV) to a scale transonic wind tunnel and to a cold burner spray is described. It is shown that diffraction limited imaging makes it possible to extend the working range of PIV systems to several meters enabling a broad variety of industrial applications and to determine particle sizes with a high magnification objective. In both cases diffraction limited imaging significantly reduced the laser energy required to form satisfactory particle images. Particle images were recorded onto 35 mm film and a CCD video camera.
A new method for extracting quantitative information from a double exposure holographic interferogram is presented. Dual reference beams are used to produce continuously variable phase differences between the two images of the object at the recording stage of the hologram. Image reconstruction at three known phase differences, via a CCD camera and digital framestore, allow new automatic image processing methods to calculate the three-dimensional surface deformation.
The present light sheet system for 3D studies of the location and structure of transonic flows employs a CCD video camera and digital frame-store which are synchronized with the pulsed laser so that the resulting images can be immediately displayed via microcomputer. These video images are digitally enhanced in order to display 3D coordinate data. An illustrative demonstration of the system is for rotating transonic-jet flow. The visualized images show instantaneous transonic flow structures.